This disclosure relates to a system and method for real time monitoring of scaling and fouling on reverse osmosis membranes.
Membrane technology is used for filtration of various liquids. This technology is particularly useful for water and wastewater treatment and water desalination as it is simple to operate, requires minimal chemical use, and produces consistent and reliable water quality.
Membrane technology may use different types of membranes for filtration. Membranes are classified according to the size of the particle that can pass through the membranes. Different types of membrane technology are categorized as microfiltration, ultrafiltration, nanofiltration and reverse osmosis. Microfiltration removes particles down to 0.1 micron, i.e. microorganisms such as Cryptosporidium and Giardia. Ultrafiltration (UF) removes particles from 0.01 to 0.1 micron. Nanofiltration (NF) removes most organic compounds, while Reverse Osmosis (RO) removes dissolved salts and metal ions.
In recent years, there has been a growing interest in the application of low pressure reverse osmosis and nanofiltraton membranes to the desalination of brackish water. In reverse osmosis and nanofiltration, a solution is passed through a semi-permeable membrane that rejects the solute and other impurities on one side (the “feed side”) and allows the pure solvent to permeate through the membrane to be obtained from the other side (the “permeate”) side.
Such membranes can provide high salt rejection and flux at low operating pressures. However, during desalination, significant levels of cations (e.g., calcium and barium) and anions (carbonate and sulfate) may concentrate near and at the membrane surface. The concentration of these ions near and at the membrane surface may exceed the solubility limits of various sparingly soluble mineral salts, such as calcium carbonate (calcite), calcium sulfate (gypsum) and barium sulfate (barite). These mineral salts may then precipitate in bulk near the membrane surface or crystallize directly onto the membrane surface.
The accumulation of these ions next to the membrane surface forms a concentration boundary layer that is referred to as the “concentration polarization” (CP) layer. The degree of concentration polarization (or CP) is often estimated using a simple film theory assumption:
                              C          ⁢                                          ⁢          P                =                                                            C                m                            -                              C                p                                                                    C                b                            -                              C                p                                              =                      exp            ⁢                          J              k                                                          (        1        )            where Cm is the concentration near the membrane, Cp is the permeate concentration, Cb is the bulk concentration, J is the permeate flux and k is the solute mass transfer coefficient. When the CP level is high, the solution near the membrane surface may become supersaturated with respect to the concentration of various mineral salts. These supersaturated conditions may lead to surface crystallization near and at the membrane surface.
The crystallization on the membrane surface is referred to as “scaling.” The deposition of organic matter on the membrane surface is called “fouling.” Scaling and fouling may lead to permeate flux decline, may adversely affect salt passage, and may cause the eventual decrease in the useful life of the membrane.
To increase the life of a membrane, it is important to prevent or suppress scale formation. Suppression of scale formation may be achieved to some degree by pH adjustment for controlling calcite scaling and by the use of antiscalants for gypsum, barite and silica scale control. However, for the prevention and suppression of scaling, early detection of scaling is desirable, as it would provide for the optimization of scale mitigation strategies. Similarly, early detection of foulant deposition (e.g. biofilms) helps in prevention and mitigation of damage due to fouling.
In conventional real-time monitoring methods and devices, scaling or damage due to scaling (or fouling) is noticeable only via flux decline and salt passage monitoring. Available scale and fouling detection methods are not capable of real time early scale detection under reverse osmosis operating conditions. Therefore, reliable methods to predict membrane scaling and fouling are important tools in the control of scaling and fouling. It is also desirable to have a device that monitors membrane scaling in real-time while placed ex-situ. This enables the external application to a reverse osmosis plant without any major adjustments in the structure and function of the plant.